Williams also experienced a last-minute switch-up to her wedding last year, when her father said he was too nervous to walk her down the aisle an hour before her ceremony.
Serena Williams is supporting her girl.
The 36-year-old tennis pro has arrived at St. George's Chapel in Windsor, England, to watch her friend, Meghan Markle, tie the knot with Prince Harry. Serena and Meghan have been friends for years, after teaming up for a game of flag football at Celebrity Beach Bowl in 2014.
The new mom, who arrived in London with her husband, Alexis Ohanian, and their 8-month-old daughter, Olympia, on Friday, was supposed to compete at the Italian Open in Rome, but pulled out of the tournament earlier this week -- freeing up her schedule to attend her friend's wedding.
Williams arrived on Saturday with Ohanian, looking stunning in a blush pink knee-length, tailored Versace gown and matching hat, a Heritage Collection necklace by Bulgari and a gold ribbon in her tightly braided hair.
Williams previewed her Royal Wedding prep on Instagram and Snapchat earlier on Saturday, showing her excitement as she got her makeup done, her eyebrows shaped and as she prepared with Ohanian and little Olympia.
"So, my friend's getting married today, I'm up super early -- well, for me," she says in an Instagram story. "I've known her for so many years and I'm so happy for her."
Meghan was rumored to be on the guest list for Serena's own wedding in New Orleans last November, but wasn't spotted at the ceremony alongside other famous guests like Beyonce and Kim Kardashian West. Just 10 days later, the former Suits star announced her engagement to Prince Harry.
And like Meghan, Serena experienced a last-minute change to her wedding. On Wednesday's episode of Being Serena, the tennis champ revealed that her father, Richard Williams, told her that he was "too nervous" to walk her down the aisle, just one hour before her wedding. Serena told her father it was OK, and walked herself down the aisle instead.
It was announced on Friday that Harry's father, Prince Charles, will be walking Meghan down the aisle at her wedding, days after her father underwent surgery and revealed he was no longer able to attend.
In an interview with ET last month, Serena gushed about her friend's upcoming nuptials, saying, "I'm obviously super happy for her. She's such a great girl and she is incredibly nice and I couldn't be happier for her."